Polymer compositions for contact lenses

ABSTRACT

Branched cycloalkyl methacrylate monomers, such as t-butylhydroxycyclohexyl methacrylate, are polymerized with hydrophilic monomers and crosslinking agents to produce compositions for shaped biomedical devices, including soft contact lenses. Such copolymers have high oxygen permeability and good mechanical properties.

This is a continuation of application Ser. No. 07/890,325, filed on May26, 1992, now abandoned, which is a continuation of Ser. No. 07/643,603filed Jan. 18, 1991 now abandoned which is a divisional of Ser. No.07/034,292 filed Sept. 2, 1987 now U.S. Pat. No. 5,006,662.

BACKGROUND OF THE INVENTION

This invention relates to polymeric compositions formed by reacting ahydrophilic monomer, a hydrophobic strengthening agent monomer, acrosslinking agent monomer and a polymerization initiator. The novelhydrophobic strengthening agent monomer provides increased strengthwithout decreasing the oxygen permeability of the polymer material.

Soft contact lens materials are made by polymerizing and crosslinkinghydrophilic monomers such as 2-hydroxyethylmethacrylate,N-vinyl-2-pyrrolidone, and combinations thereof. The polymers producedby polymerizing these hydrophilic monomers exhibit significanthydrophilic character themselves, and are capable of absorbing asignificant amount of water in their polymeric matrices. Due to theirability to absorb water, these polymers are often referred to as"hydrogels". These hydrogels are optically clear and, due to their highlevels of water of hydration, are particularly useful materials formaking soft contact lenses. However, the high levels of water ofhydration of hydrogels contributes to their relative lack of physicalstrength which results in hydrogel contact lenses being relatively easyto tear.

Various hydrophobic monomers have been copolymerized with thesehydrophilic monomers in order to obtain polymers with improved physicalstrength. Such hydrophobic monomers include styrene, and variousacrylates and methacrylates such as methylmethacrylate,isobornylmethacrylate, and t-butylcyclohexylmethacrylate.

For instance, Chromecek teaches that t-butylcyclohexylmethacrylate is aparticularly useful monomer when copolymerized with2-hydroxyethylmethylacrylate in U.S. Pat. No. 4,436,887. However, whilethese state of the art hydrophobic monomers do increase the physicalstrength of hydrogel polymers, they also produce polymers with lowerlevels of water of hydration than unmodified hydrogels. It is an objectof the present invention to provide polymeric materials with increasedphysical strength and high levels of water of hydration.

SUMMARY OF THE INVENTION

According to the present invention it has been found that certainhydrophobic monomers act as strengthening agents when copolymerized withhydrophilic monomers such as 2-hydroxyethylmethacrylate,N-vinyl-2-pyrrolidone and the like. These hydrophobic strengtheningagent monomers are represented by the general formula ##STR1## wherein:R³ is H or CH₃ ;

R² is an divalent alkylene radical chosen from the group consisting of--CH₂ --, --CHOH--, and --CHR⁴ where R⁴ denotes a branched alkyl groupwith 3 to 8 carbon atoms and preferably with 4 to 6 carbon atoms; and

R¹ denotes O or NH;

the R² radicals are chosen so that at least one --CHOH-- radical ispresent, at least one --CHR⁴ -- radical is present and is separated fromthe --CHOH radical by at least one --CH₂ -- radical; and n is 4, 5, 6,or 7. Copolymers of the present invention's hydrophobic monomers withhydrophilic monomers used in the production of hydrogel materialsexhibit substantial hydrophilic character, and substantially greatermechanical strength then state of the art hydrogel materials.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to hydrophobic monomers which can becopolymerized with hydrophilic monomers used in making soft contactlenses in order to provide polymeric materials with improved strength.These materials may also contain crosslinking agents and polymerizationinitiators. Thus, the present invention also relates to the product ofpolymerizing hydrophilic monomers with hydrophobic monomer strengtheningagents and, optimally, crosslinking agents and polymerizationinitiators.

By weight the hydrophilic monomers comprise from 35 to 90 weight percentof a mixture useful in forming soft contact lenses. The novelhydrophobic strengthening agents comprise from 10 to 65 weight percent,the crosslinking agent comprises from 0 to 5 weight percent and thepolymerization initiator comprises from 0 to 5 weight percent of themixture to be polymerized.

The novel hydrophobic strengthening agent monomers of the presentinvention useful in soft contact lens compositions are represented bythe general formula ##STR2## wherein:, R¹ is O or NH;

R² is chosen from the group of radicals consisting of --CH₂ --,--CHOH--, and --CHR⁴ -- where R⁴ denotes a branched alkyl radical with 3to 8 carbon atoms and preferably 4 to 6 carbon atoms;

R³ is H or CH₃ ; and

n is 4, 5, 6 or 7.

At least one R² radical is --CHOH-- and at least one R² radical is--CHR-- and these two radicals are separated by at least one --CH₂ --radical.

These branched alkyl hydroxy cycloalkyl acrylates, or the correspondingmethacrylates, acrylamides and methacrylamides are effectivestrengthening agents in soft contact lens polymeric compositions. Theresulting polymers are optically clear and possess a combination of highlevels of water of hydration and good mechanical properties.

Non-limiting examples of the hydrophobic strengthening agent monomers ofthe present invention include: ##STR3##

The hydrophilic monomers useful in the present invention are those knownin the art to be useful in making soft contact lenses. The following arenon-limiting examples of hydrophilic monomers useful in contact lensformulations of this invention: ##STR4##

The crosslinking agents which are useful in the present invention arethose crosslinkers known in the art to be useful in making soft contactlenses. The following are non-limiting examples of crosslinking agentsuseful in contact lens formulations of this invention: ##STR5##

Various mixtures of hydrophilic monomers, hydrophobic strengtheningagent monomers, and optionally crosslinking agents can be used toproduce polymeric materials useful in contact lenses. The followingtable provides several examples of such useful mixtures. The table doesnot exhaust the list of examples which are within the scope of theinvention.

                  TABLE I                                                         ______________________________________                                        Hydrophilic   Strengthening Crosslinking                                      Monomer(s)    Agent Monomer(s)                                                                            Monomer(s)                                        ______________________________________                                        hydroxyethlmethacrylate                                                                     4-t-butyl-2   ethyleneglycol                                    (HEMA)        hydroxy-      dimethacrylate                                                  cyclohexyl-   (EGDMA)                                                         methacrylate                                                                  (TBE)                                                           HEMA          4-t-butyl-2-  EGDMA                                                           hydroxy-                                                                      cyclohexyl-                                                                   methacrylamide                                                                (TBA)                                                           HEMA          octafluoropentyl                                                                            EGDMA                                                           methacrylate                                                                  (OFPMA)/TBE                                                     HEMA/N-methacryloyl                                                                         TBA           EGDMA                                             glycine                                                                       N-vinyl 2-pyrrolidone                                                                       TBE           DVEU                                              (NVP)                                                                         NVP           TBE           DVEU/EGDMA                                        NVP           TBE           Allyl                                                                         methacrylate                                      Glyceryl methacrylate                                                                       TBE           EGDMA                                             (GM)                                                                          HEMA/GM       TBE           EGDMA                                             NVP           OFPMA/TBE     DVEU                                              ______________________________________                                    

The most preferred contact lenses of the instant invention have anoxygen transport rate of at least about 2×10⁻⁶ cm³ /(sec. cm² atm.), arehydrolytically stable, biologically inert, transparent, resilient, andhave a softness preferably of about 60 or below on the Shore hardness Ascale when hydrated. The more preferred materials have a Shore hardnessbetween 25 to 35 on the A scale. The tensile modulus of elasticity ofthese hydrated polymers is at least about 50 g/mm², preferably fromabout 75 g/mm² to about 100 g/mm² and the tear strength is at leastabout 2.0 g/mm thickness, preferably from about 2.0 g/mm to about 250g/mm thickness. High tensile modulus of elasticity is desirable forstrength and durability. High tear strength is desirable in order toprevent damage to the contact lens (1) due to tearing during patientuse, i.e., the removing and the placing of the lens in the eye, and (2)to prevent damage to the lens during cleansing and disinfecting.

Soft contact lenses, made from the polymers of the instant invention,can be made large enough to cover the entire cornea of the eye,resulting in more comfort because of their good oxygen permeability.Conventional hard contact lenses have to be made smaller due to theirpoor oxygen transportability. Furthermore, the larger the lenses, theeasier it is to locate the optical center of the lenses. The larger thelens the easier it is to maintain the optical axis which is required inmaking special lenses for people with astigmatism. Another advantage ofthe preferred soft lenses of the instant invention is the instantpreferred soft lenses have a softness similar to HEMA lenses but aremore oxygen permeable. HEMA lenses are not oxygen permeable or capableof transporting oxygen to a degree necessary to meet all therequirements of the human cornea, especially for extended wearapplications.

While the polymers of the instant invention can be used to preparecontact lenses, these polymers can also be used to make other shapedarticles for use in biomedical applications. These polymers can be usedto make biomedical devices such as dialyzer diaphragms used inartificial kidneys and other biomedical implants, such as disclosed inWichterle, U.S. Pat. No. 2,976,576 and Wichterle U.S. Pat. No.3,220,960. The instant polymers can be used in preparing therapeuticbandages as disclosed in Shephard, U.S. Pat. No. 3,438,043. The instantpolymers can also be used in preparing such medical surgical devices asheart valves, vessel substitutes, intrauterine devices, membranes,films, dialyzer diaphragms, catheters, mouth guards, denture liners andother such devices as disclosed in Shephard U.S. Pat. No. 3,520,949 andShephard U.S. Pat. No. 3,618,231. The instant polymers can be used tomodify collagen to make blood vessels, urinary bladders and other suchdevices as disclosed in Kliment U.S. Pat. No. 3,563,925. The instantpolymers can be used to make catheters as disclosed in Shepherd U.S.Pat. No. 3,566,874. The polymers can be used in semipermeable sheets fordialysis, artificial dentures and all of such disclosures as set forthin Stoy U.S. Pat. No. 3,607,848. The instant polymers can be used inophthalmic prostheses and all other uses disclosed in Wichterle U.S.Pat. No. 3,679,504.

The shaped articles for use in biomedical applications made from thepolymers of this invention have physiochemical properties rendering themsuitable for prolonged contact with living tissue, blood or the mucousmembrane such as surgical implants, blood dialysis devices, bloodvessels, artificial ureters, artificial breast tissue and membranesintended to come in contact with body fluids outside of the body, forexample, membranes for kidney dialysis and heart/lung machines, and thelike. For example, it is known that blood is rapidly damaged in contactwith artificial surfaces. The design of a synthetic surface which isantithrombogenic and nonhemolytic is necessary for prosthesis anddevices which contact blood. The polymers of the present inventionprovide such surfaces.

The polymers disclosed herein can be boiled and/or autoclaved in waterwithout being damaged, whereby sterilization may be achieved. Thus, anarticle formed from the instant polymers disclosed herein may be used insurgery where an article compatible with living tissue or with themucous membrane is required.

The polymers disclosed herein can be used in shaped articles in hydratedstates and can contain high amounts of water of hydration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The utility of the compositions of this invention as soft contact lensformulations is illustrated in the following examples. Examples 1, 3, 6,7, 8 (in part), and 9 illustrate the use of4-t-butyl-2-hydroxycyclohexyl methacrylate (TBE), a preferredstrengthening agent of this invention. Example 2 illustrates the use of4-t-butyl-2-hydroxy cylohexyl methacrylamide, which is another preferredstrengthening agent of this invention. For comparative purposes,examples 4, 5, 8 (in part) and 10 illustrate the use oft-butylcyclohexyl methacrylate, which is a state of the artstrengthening agent outside the scope of this invention, and whichdiffers from the strengthening agent of the invention only in theabsence of a hydroxy group on the cycohexyl substituent of the monomer.

EXAMPLE 1

A mixture was made containing 77.0 g of glycerylmethacylate, 22.5g of4-t-butyl-2-hydroxycyclohexyl methacrylate, and 0.5g of ethyleneglycoldimethacrylate. To this mixture was added 0.5g of benzoin methyl ether,an ultra violet induced polymerization initiator. The solution was castbetween glass plates separated by a Teflon (DuPont registered trademark)perfluoro polymer gasket 0.3 mm thick and cured. After curing, the filmwas released from the glass plates and hydrated and extracted in hotdistilled water for 4 hours.

Next the film was placed in a borate buffered saline solution fortesting. The resultant material was optically clear and had a watercontent of 53% and an oxygen permeability of 18.0×10⁻¹¹ cm³ cm/(sec. cm²mm mmHg). The mechanical properties were measured according to thefollowing test methods and gave the following results:

    ______________________________________                                        PROPERTIES (RESULT)    TEST METHOD                                            ______________________________________                                        Young's modulus of elasticity (60 g/mm.sup.2)                                                        ASTM-D 1708                                            Tensile strength (84 g/mm.sup.2)                                                                     ASTM-D 1708                                            % Elongation (164%)    ASTM-D 1708                                            Tear Initiation (3.8 g/mm)                                                                           ASTM-D 1938                                            ______________________________________                                    

EXAMPLE 2

A mixture of 83.5 parts by weight of 2-hydroxyethyl methacrylate, 10parts of 4-t-buty-2-hydroxycyclohexyl methacrylamide, 6 parts ofn-methacryloylglycine, and 0.5 parts of ethylene glycol dimethacrylatewas made. To this mixture was added 0.17 parts by weight of benzoinmethyl ether, a U.V. initiator, and 10 parts by weight of ethyleneglycol as a diluent. The mixture was placed between glass plates andcured as described in Example 1. The resultant polymer when hydrated hada water content of 65% and an oxygen permeability of 33×10⁻¹¹ cm³cm/(sec. cm² mmHg). The mechanical properties measured were thefollowing:

    ______________________________________                                                               MEASURED                                               PROPERTY               VALUE                                                  ______________________________________                                        Young's modulus of elasticity                                                                        44     g/mm.sup.2                                      Tensile strength       34     g/mm.sup.2                                      % Elongation           139                                                    Tear Initiation        2.0    g/mm                                            ______________________________________                                    

EXAMPLE 3

A mixture was prepared using 89.9 parts of n-vinyl-2-pyrrolidinone, 10parts of 4-t-butyl-2-hydroxycyclohexyl methacrylate, 0.1 parts ofdivinyl ethylene urea and 1 part of 2,2-azobis(isobutyronitrile) and 0.1part of 2--2 azobis(2,4-dimethyi-4-methoxyvaleronitrile). The solutionwas purged with nitrogen for 10 minutes and then poured intopolypropylene tubes having a diameter of 18 mm and a length of 300 mm.The tubes were closed then immersed in a constant temperature bath andheated to 32° C. for 72 hours, then raised to 45° C. and held for 48hours, and then raised to and held at 60° C. for 48 hours. The resultantrod was removed from the water bath and placed in an oven at 90° C. for2 hours for post curing. Cylinders were cut from the rod and annealed byheating for 1 hour at 110° C. and cooled down very slowly. From thesecylinders, flat discs or lenses were machined by conventionaltechniques. The lenses and discs were placed in hot distilled water forfour hours to hydrate and extract solvents from them. Last, the lensesand discs were placed in isotonic saline for testing. The materials wereoptically clear, soft, tough and biologically compatible with eyetissue. Preliminary clinical testing of lenses was favorable. The discshave water contents of 83% and oxygen permeabilities of 59×10⁻¹¹ cm³cm/(sec cm² mmHg). The mechanical properties of the materials measuredwere:

    ______________________________________                                        PROPERTY           MEASURED VALUE                                             ______________________________________                                        Young's modulus of elasticity                                                                    60        g/mm.sup.2                                       Tensile strength   97        g/mm.sup.2                                       % Elongation       223                                                        Tear Initiation    6.2       g/mm                                             ______________________________________                                    

EXAMPLE 4

Mixtures of 40 parts of 2-hydroxyethylmethacrylate, 60 parts of glycerylmethacrylate and X parts of 4-t-butylcyclohexyl methacrylate, 0.5 partsof ethylene glycoldimethacrylate and 0.5 parts of benzoin methyl ether,where X was 5, 7, and 10 parts, were made. Films were cast by placingthe mixtures between glass plates as described in Example 1. Afterhydration, the films were not optically clear.

EXAMPLE 5:

Example 4 was repeated except 4-t-butyl-2-hydroxycyclohexyl methacrylatewas used in place of t-butylcyclohexyl methacrylate. The resultant filmswere transparent and tough. Up to 30 parts of the4-t-butyl-2-hydroxycyclohexyl methacrylate was added to the mixture andthe resultant films were optically clear. This demonstrates thesolubilizing aspect of this strengthening monomer and shows that it canbe used in higher concentrations than state of the art strengthening orstrengthening agents.

COMPARATIVE EXAMPLE USING t-BUTYLHYDROXYCYCLOHEXYL METHACRYLATE ANDt-BUTYLCYCLOHEXYL METHACRYLATE EXAMPLE 6

Films were made by the methods of Example 1 using the followingcompositions:

A) 2-Hydroxyethyl methacrylate (EM) 83.4 parts, n-methacryloylglycine(MG) 6 parts, ethyleneglycoldimethaerylate (EGDMA) 0.6 parts,t-butylcyclohexyl-methacrylate (t-BCM) 10.0 parts, and benzoin methylether (BME) 0.17 parts by total weight;

B) HEMA 83.5 parts, MG 6.0 parts, 10 parts t-butylhydroxycyclohexylmethacrylate (TBE) and 0.5 parts EGDMA. To this mixture was added 0.17parts by weight BME and 15 parts ethylene glycol as a diluent;

C) HEMA 73.5 parts, MG 6 parts, 20 parts TBE and 0.5 parts EGDMA. Tothis mixture was added 0.17 parts by weight of BME and 15 parts ofethylene glycol as a diluent; and

D) HEMA 63.5 parts, MG 6.0 parts, TBE 30 parts and 0.5 parts EGDMA. Tothis mixture was added 0.17 parts BME and 15 parts ethylene glycol.

The physical properties of the cast films were measured and are reportedin Table 2.

                  TABLE 2                                                         ______________________________________                                                      FORMULATION                                                     PROPERTY        A       B        C    D                                       ______________________________________                                        Young's modulus of                                                                            142     44       97   5,860                                   elasticity g/m.sup.2                                                          Tensile strength g/m.sup.2                                                                    65      40       49   293                                     % Elongation    75      166      113  95                                      Tear Initiation g/mm                                                                          51      25       78   66                                      % Water         56      65       58   49                                      Oxygen Permeability DK                                                                          16.2  34       27     16.2                                  ______________________________________                                    

The results show substantially more TBE can be added without affectingthe oxygen permeability of the resulting polymer. The upper solubilitylimit for T-BCM was found to be about 15 parts before films were notoptically clear on hydration. Although more TBE is required to obtainthe same toughness as obtained by the strengthening agents of thepresent invention, this does not impact the oxygen permeability.

COMPARATIVE EXAMPLE USING CYCLOHEXYLMETHACRYLATE, t-BUTYLCYCLOHEXYLMETHACRYLATE AND TBE. EXAMPLE 7

A mixture was made using 84.7 parts of n-vinyl-2-pyrrolidinone, 15 partsof TBE and 0.3 parts of EGDMA. To this mixture was added 0.5 parts of2,2-azobis (isobutyronitrile). The mixture was cast between glass platesas described in Example 1. The samples were cured in an oven for 2 hoursat 60° C. 1 hour at 80° C. and 1 hour at 100° C. The film was releasedand boiled in distilled water for 4 hours, then put in a saline solutionand tested. The properties of the film are reported below:

    ______________________________________                                        PROPERTY           MEASURED VALUE                                             ______________________________________                                        Tangent modulus of elasticity                                                                    1100      g/mm.sup.2                                       Tensile strength   220       g/mm.sup.2                                       % Elongation       88                                                         Initial Tear       19.9      g/mm                                             % Water            73.8                                                       Oxygen Permeability                                                                              40                                                         ______________________________________                                    

EXAMPLE 8

Example 7 was repeated except cyclohexl methacrylate andt-butyl-cyclohexyl methacrylate were used to replace the TBE. The filmwas clear on casting, but immediately turned cloudy on hydration. Thatis, the material was not optically clear. This supports the conclusionthat the novel monomer acts to toughen the material and to solubilizethe components in the polymer.

Examples 1, 2, 3, 6, and 7 show that polymer compositions employing thestrengthening agents of this invention are optically clear, possess highwater contents, good oxygen permeability levels and good mechanicalproperties. The best results of this invention were obtained in Example3, wherein the hydrophilic agent is N-vinyl-2-pyrrolidinone. Theformulation of Example 3 provides a remarkably high water level andoxygen permeability while retaining unusually good mechanicalproperties. Based upon this data, the polymeric hydrogels of thisinvention can have water of hydration contents of 60, 70 or 80 weightpercent, or more.

Polymeric soft contact lens hydrogels of the prior art can achievemoderately elevated water of hydration levels while retaining goodmechanical properties but can only achieve highly elevated water ofhydration levels at the expense of good mechanical properties. Forexample, many such hydrogels with water of hydration levels of about 70weight percent retain some mechanical properties. However, soft contactlens polymeric hydrogels of the prior art having water of hydrationlevels of 80 percent, or more, exhibit sharply reduced mechanicalproperties. In contrast to the prior art, the present invention iscapable of producing polymeric soft contact lens hydrogels having waterof hydration levels of 80, 82 or 85 weight percent, or more, whileretaining good mechanical properties including Young's modulus ofelasticity of at least 50 g/mm², generally; at least 75 g/mm²,preferably; and at least 100 g/mm², most preferably; and an initial tearresistance of at least 2 g/mm, generally; at least 7 g/mm, preferably;and at least 10 g/mm, most preferably.

Table 1 shows the results of tests made with various formulations ofN-vinyl-2-pyrrolidinone (NVP), 4-t-butyl, 2-hydroxycyclohexylmethacrylate (TBE) and ethylene glycol dimethacrylate (EGDMA)crosslinker. The formulations are given in weight percent.

                  TABLE 1                                                         ______________________________________                                                        Modulus of                                                                              Tensile                                             Monomer-Wt. Percent                                                                           Elasticity                                                                              Strength Elongation                                 NVP    TBE     EGDMA    g/mm.sup.2                                                                            g/mm.sup.2                                                                           %                                      ______________________________________                                        84.642 15.058  0.299    1100    220    88.5                                   84.391 15.001  0.604    1530    214    65.0                                   84.103 14.993  0.900    1990    231    51.9                                   87.176 12.497  0.321    604     130    83.6                                   86,868 12.500  0.632    794     157    66.4                                   86.587 12.516  0.898    868     173    66.4                                   89.672 10.003  0.325    178     83.6   97.4                                   89.381 10.011  0.608    313     94.9   72.3                                   89.067 10.008  0.912    448     106.0  57.3                                   89,668 10.006  0.300    192     83.5   83.6                                   91,677 8.017   0.306    835     48.2   81.6                                   93,673 6.011   0.310    20.7    22.6   73.2                                   89,723 10.048  0.276    208     78.4   81.5                                   90,727 9.038   0.282    134     69.1   95.5                                   91.762 8.007   0.266    75.5    44.5   79.8                                   91.653 8.005   0.319    69      43.9   84.3                                   91.354 8.031   0.602    156     68.2   79.4                                   91.101 7.999   0.854    217     77.9   67.5                                   89,607 10.074  0.299    174     77.7   97.0                                   89,060 10.039  0.602    308     93.3   74.8                                   89.137 8.012   0.847    200     68.4   62.3                                   93.697 6.003   0.299    21.2    13.4   51.1                                   93.051 10.029  0.597    26.6    92.0   81.4                                   93.084 10.008  0.892    36.2    103.0  65.0                                   ______________________________________                                    

Table 2 shows the results of tests made to show water of hydration withvarious formulations of N-vinylpyrrolidinone (NVP), 4-t-butyl,2-hydroxycyclohexyl methacrylate (TBE), and divinylethylene urea (DVEU).The results also show the amount of monomer and oligomers extractablefrom the polymer.

                  TABLE 2                                                         ______________________________________                                        WEIGHT PERCENT       WATER    EXTRACT                                         NVP   TBE    DVEU    PLASTIZER WT. %  WT. %                                   ______________________________________                                        77.5  11     1.5     10        74.7   5.66                                    65.5  14     0.5     20        75.8   7.42                                    85.5  14     0.5      0        77.1   15.8                                    84.5  14     1.5      0        71.7   11.7                                    90.5   8     1.5      0        77.5   14.6                                    71.5   8     0.5     20        82.6   6.9                                     91.5   8     0.5      0        83.1   11.4                                    68.0  11     1.0     20        74.5   2.5                                     81.0   8     1.0     10        80.2   2.7                                     70.5   8     1.5     20        76.4   2.1                                     64.5  14     1.5     20        67.6   -0.4                                    85.0  14     1.0      0        73.9   6.1                                     88.5  11     0.5      0        80.0   12.1                                    86.7  13     0.3      0        80.1   11.9                                    71.0   8     1.0     20        79.0   0.08                                    65.0  14     1.0     20        72.3   2.7                                     75.5  14     0.5     10        76.3   4.4                                     86.7  13     0.3      0        79.7   14.5                                    70.5  14     0.5     15        76.7   6.7                                     70.0  14     1.0     15        73.9   4.4                                     82.5  11     1.5      5        71.5   -2.2                                    86.0   8     1.0      5        77.4   -1.7                                    81.5   8     0.5     10        81.9   2.9                                     75.5  14     0.5     10        75.7   4.2                                     80.5   8     1.5     10        74.7   -3.3                                    74.4  14     1.5     10        69.0   -1.7                                    78.0  11     1.0     10        76.3   3.4                                     80.5  14     0.5      5        76.4   9.2                                     ______________________________________                                    

Table 2 shows the high water of hydration levels achieved with thetested polymeric composition of this invention. All of the polymersshowed hydration levels above 60 weight percent, most of the polymersshowed hydration levels above 70 or 75 percent and some showed hydrationlevels above 80 percent. High levels of water of hydration are conduciveto good oxygen permeability.

Table 3 shows the results of tests performed with two different polymerformulations of this invention. The first formulation was prepared fromN-vinylpyrrolidinone (NVP), t-butyl 2-hydroxycyclohexyl methacrylate anddivinylethylene urea (DVEU). The second formulation was prepared fromN-vinyl-2-pyrrolidinone (NVP), t-butyl, 2-hydroxycyclohexyl methacrylate(TBE), divinylethylene urea (DVEU) and ethylene glycol dimethacrylate(EGDMA).

                                      TABLE 3                                     __________________________________________________________________________                                    Tear,                                                    Modulus of                                                                          Tensile                                                                            Elon-                                                                              Tear,                                                                              Propa-                                                                             H.sub.2 O,                                          Elasticity                                                                          Strength                                                                           gation                                                                             Initial                                                                            gation                                                                             Wgt.                                     Formulation                                                                              (g/mm.sup.2)                                                                        g/mm.sup.2                                                                         %    (g/mm)                                                                             (g/mm)                                                                             %                                        __________________________________________________________________________    NVP/TBE/DVEU                                                                  (Wgt. %)                                                                      89.7/10/.3 48.7  58.1 233.0                                                                              5.0  82.5                                          86.7/13/.3 117.0 89.5 220.0                                                                              7.7  80.2                                          85.7/14/.3 132.0 100.0                                                                              238.0                                                                              10.9 79.1                                          83.7/16/.3 218.0 124.0                                                                              217.0                                                                              15.9 79.5                                          83.7/16/.3 283.0 132.0                                                                              234.0                                                                              15.4 75.7                                          NVP/TBE/DVEU/                                                                 EGDMA                                                                         (Wgt. %)                                                                      89.5/10/.25/.25  75.2 87.6 228.0                                                                              6.2  85.4                                     90/9/0/.3  129.0 97.0 111.0                                                                              5.9  82.7                                          __________________________________________________________________________

Table 3 shows that the formulations of this invention which employN-vinyl pyrrolidone as the hydrophilic monomer can achieve water levelsof at least 85 weight percent. Formulations which employt-butylhydroxycyclohexyl methacrylamide as the strengthening agent canachieve water levels above 79 weight percent. These high water levelsare achieved while maintaining good mechanical properties.

EXAMPLE 9

The following three synthetic procedures describe the synthesis of twointermediates used to make TBE, and the TBE synthesis.

EXAMPLE 9A Synthesis of 4-t-butylcyclohexene

4-t-butylcyclohexene, an intermediate in the synthesis of4-t-butylhydroxycyclohexyl methacrylate, is prepared by reacting4-t-buylcyclohexanol, a commercially available material, with phosphoricacid. A mixture of 4-t-butylcyclohexanol and phosporic acid is made byslow addition of the acid to the 4-t-butylcyclohexanol. This mixture isrefluxed at about 70° C. until the reaction is complete. The reactionmixture is extracted with an organic solvent such as ethylacetate, driedover Mg SO₄ and filtered. The theoretical reaction is: ##STR6##

EXAMPLE 9B Synthesis of 4-t-butyl-2-epoxycyclohezene

4-t-butyl-2-epoxycyclohexane, an intermediate in the synthesis of4-t-butylhydroxycyclohexyl methacrylate, is prepared by reacting a4-t-butylcyclohexane with m-chloroperoxybenzoic acid in ethylacetatebelow 5° C. until mixing is complete, and then at room temperature fortwo hours. The reaction mixture is washed with aqueous NA₂ CO₃, driedover MgSO₄, and filtered. The theoretical reaction is: ##STR7##

EXAMPLE 9C

Synthesis of 4-t-butyl-hydroxycyclohexyl methacrylate

4-t-butyl-2-epoxycyclohexane and methacrylic acid are reacted in thepresence of CuCl, BHT and TEA for about 2 hours at about 90° C. Lowboiling components are distilled off and the final product is purifiedby further distillation. The theoretical reaction is: ##STR8##

What is claimed is:
 1. The polymerization product of a mixturecomprising: 35 to 90 parts by weight of hydrophilic monomer; 0.1 to 5parts by weight of crosslinking agent; 0.01 to 5 parts by weight ofcatalyst; and 10 to 65 parts by weight of strengthening agent, whereinsaid strengthening agent comprises a compound of the generalformula:wherein: R¹ is O or NH; R² is a divalent alkylene radical chosenfrom the group consisting of --CH₂ --, --CHOH-- and --CHR⁴ -- where R⁴is a branched alkyl group with 3 to 8 carbon atoms, preferably with 4 to6 carbon atoms; and R³ is H or CH₃ ; and wherein at least one R² radicalis --CHOH-- and at least one R² radical is --CHR-- and are separated byat least one --CH₂ --radical, and n is 4, 5, 6 or
 7. 2. Thepolymerization product of claim 1 wherein R¹ is O.
 3. The polymerizationproduct of claim 1 wherein R¹ is NH.
 4. The polymerization product ofclaim 1 wherein R² is a branched C₄ to C₆ alkyl group.
 5. Thepolymerization product of claim 1 wherein R² is an isobutyl group. 6.The polymerization product of claim 1 wherein n is
 5. 7. Thepolymerization product of claim 1 wherein n is
 6. 8. The polymerizationproduct of claim 1 wherein R³ is H.
 9. The polymerization product ofclaim 1 wherein R³ is CH₃.
 10. The polymerization product of claim 1wherein said strengthening agent comprises 4-t-butyl,2-hydroxycyclohexylmethacrylate.
 11. The polymerization product of claim 1 wherein saidstrengthening agent comprises 4-t,-butyl,2-hydroxy-cyclohexylmethacrylamide.
 12. The polymerization product of claim 1 wherein saidhydrophilic monomer is selected from the group comprising hydroxyethylmethacrylate, n-methacryloyl amino hydroxyethane, N-vinylpyrrolidone,glyceryl methacrylate and n-methacryloyl glycene.
 13. The polymerizationproduct of claim 1 wherein said hydrophilic monomer isN-vinylpyrrolidone.
 14. The polymerization product of claim 1 whereinsaid hydrophilic monomer is N-vinylpyrrolidone and said strengtheningagent is 4-t-butyl,2-hydroxycyclohexyl methadrylate.